Polyethylene is divided into high density (HDPE, density 0.941 g/cm3 or greater), medium density (MDPE, density from 0.926 to 0.940 g/cm3), low density (LDPE, density from 0.910 to 0.925 g/cm3) and linear low density polyethylene (LLDPE, density from 0.910 to 0.925 g/cm3). See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials. Linear polyethylene, including HDPE, MDPE, and LLDPE, is generally made by coordination catalysts such as Ziegler-Natta and single-site catalysts, while branched polyethylene, LDPE, is made by free radical polymerization at high pressure. For linear polyethylene, the density varies with the quantity of comonomers used with ethylene. The comonomer forms short-chain branches along the ethylene backbone, and since branches create separation between the ethylene backbone, the greater the quantity of comonomer, the lower the density of the polymer. On the other hand, the density of branched polyethylene depends on the closeness and regularity of the packing of the long-chain branches, and is varied through changes in the reactor pressure and heat. Therefore, the density of branched polyethylene cannot be broadly varied and HDPE and MDPE resins are generally not made by high pressure, free radical polymerization.
Compared with LLPDE, LDPE has a unique combination of properties including high shrink characteristics, processability, and film transparency. These unique attributes of LDPE largely result from its long-chain branched structure. MDPE resins have been made recently by high pressure, free radical polymerization. See U.S. Pat. Appl. Pub. No. 2004/0054097. Unlike the traditional MDPE, which are copolymers of ethylene with α-olefin comonomers, these new MDPE resins are highly branched ethylene homopolymers. They combine the attributes of traditional MDPE and LDPE. However, these new MDPE resins are more expensive to make because they require unique polymerization conditions and they are currently used in limited areas such as medical and cosmetic applications. The new MDPE resins made by high pressure, free radical polymerization, have many unique properties and they offer new opportunities for improvement of polyethylene films.